1. Field of the Invention
The present invention relates generally to an engine/transmission system and more specifically to such a system wherein the engine is supercharged using either one, or both of a supercharger and a turbocharger.
2. Description of the Prior Art
JP-A-63-17131 and JP-A-61-113526 disclose examples of arrangements wherein engines are combined with automatic transmissions. However, in these arrangements the engine torque and the transmission shifting are controlled independently of one another. As a result, during transmission shifting, if engine torque control is either initiated or stopped, a relatively large shift shock tends to be produced and leads to the situation wherein erratic shift shocks tend to occur.
In order to overcome this problem it was proposed to provide communication between the transmission and engine torque control units and to limit the initiation and termination of engine torque control during transmission shifting.
However, this arrangement suffers from the drawback that this arrangement controls only the ignition timing or air-fuel ratio and ignores the effect of a supercharging device such as an exhaust gas driven turbocharger or mechanically driven supercharger. Thus, during upshifts which are induced under minimum engine load conditions (non-depressed accelerator pedal conditions) still a wide variation in shift shock are encountered.
That is to say, in an engine/transmission system wherein the engine is supercharged, the amount of shift shock will vary depending on the presence or absence of supercharge pressure at the time an upshift is produced under minimal engine load. By way of example, even in the case wherein the induction system is synchronously controlled with the other torque control measures, the actual amount of air which is supplied to the engine varies. That is to say, the rate at which the engine speed drops (dashpot rate) is effected by the supercharge pressure in a manner wherein under some conditions it is rapid and slow under others. This variation in engine speed is basic cause of the shift shock generation.
It should be noted that depending on the presence or absence of supercharge pressure, until the engine speed reaches a predetermined level (e.g. 3000 rpm) the supercharge produces relatively little effect. However, after this level is exceeded the effect of the supercharge pressure increases dramatically. For example, in the case where the external load on the engine is high and the accelerator pedal depression changes from a state wherein a high supercharge pressure is induced in the induction manifold, to a fully released state, even though the throttle valve assumes a fully closed state, the supercharge pressure tends to remain. On the other hand, if the accelerator pedal moves from a half-throttle position to a fully released one while the engine load is low and the supercharging level is low, the induction pressure soon becomes negative.